The present invention relates to a spectrometer sampling system, and more specifically, to frequency demodulated single detector for a Fourier transform spectrometer (FTS) sampling system.
An FTS may be used by measuring a spectral radiation signal through a Michelson interferometer, for example. As shown in FIG. 1, a FTS data acquisition system 10 for acquiring FTS data from a Michelson interferometer is provided. In FIG. 1, a laser signal 11 and a spectral signal 12 are input into a spectrometer 14 controlled by a drive controller 13. The respective signals 11 and 12 are then guided to separate detectors (i.e., a laser detector 15 and a spectral signal detector 16). The laser signal 11 at the signal conditioner 18 is then input into the A/D converter 17 and used to trigger the A/D converter 17, which in turn outputs the velocity corrected spectral data 19. The Fourier transform of the velocity corrected spectral data 19 may display absorption lines.
In another FTS data acquisition system, an A/D converter is triggered by a clock as shown in FIG. 2. FIG. 2 is a diagram illustrating another FTS data acquisition system. As shown in the FTS data acquisition system 20, a laser signal 21 and a spectral signal 22 are supplied to a spectrometer 24 controlled by a drive controller 23. These signals 21 and 22 are separately detected via separate detectors (e.g., a laser detector 25 and a spectral detector 26). The laser signal 21 is guided to the laser detector 25 and instead of triggering an A/D converter 27 directly, it triggers a fringe counter 29 (i.e., an event counter) and the A/D converter 27 is triggered by a clock 28 and outputs uncorrected spectral data 30. Several problems are associated with this system, for example, one issue is non-linear sampling in space. Typically, in an FTS, a reflecting mirror is moved along a slide at constant speed during the scan, however since there may be velocity variations the data needs to be linearized (i.e., re-sampled) with respect to a known reference using a metrology laser, for example. Therefore, after triggering the fringe counter 29, the fringe timing information 31 is then stored to a separate file and used along with the uncorrected spectral data 30 to performed velocity correction calculations (i.e., demodulation (at element 32)) of the uncorrected spectral data 30 during post processing as shown in FIG. 3. The post processing produces velocity corrected spectral data 33. Thus, as shown in system 20 of FIG. 2, there is a need for additional hardware (e.g., the event counter 29), post processing, and tuning for performing velocity correction of the uncorrected spectral data 30.
Therefore, it is desirable to have an FTS data acquisition system capable of obtaining the FTS data while being able to avoid the need for tuning and any additional hardware.